Re: Richard circular print washer
From: Richard Knoppow <dickburk@ix.netcom.com> Subject: Re: Richard circular print washer Date: Sun, 10 Aug 2008 17:36:43 -0700 > Removing hypo from film or paper requires a rapid > change of water at the surfaces. The emulsion of both > washes out by a diffusion process so the rate of > exchange depends on the difference in concentration of > hypo in the emulsion vs the water. Removal of residual fixer is not a mere diffusion process, a point, which a lot of amateur chemists make mistake in popular magazines and textbooks. Washing process is a sequence of two processes, the first being desorption and the second, diffusion. The reason why fiber based papers require longer washing time is partly because the baryta sizing layer has strong adsorption of thiosulfate, thereby requiring more energy to desorb, and also more diffusion distance for the desorbed thiosulfate to come out of the print material. > The greater the difference the faster the hypo > diffuses. This also causes the washing rate to slow even > when a very good supply of fresh water is supplied because > the concentration in the emulsion becomes less as washing > progresses. Not quite. In a long wash, you will see a superposition of two exponential processes. One having a short time constant, for the diffusion part. The other has a longer time constant for desorption. The reason why the rate of washing process goes down after several minutes of initial washing is because it is desorption-limited process at that time. Use of washing aid can help that very much. The level of thiosulfate in the wash water after a couple of minutes of wash is low enough that you can soak fresh dry paper in that water and the residual thiosulfate level come to a level way below the ISO archival standard. Therefore, there is no need to waste a lot of fresh water; it suffices to use a circulator pump to create adequate flow to facilitate diffusion process while the slow desorption dominates. Again, use of washing aid saves time and eliminates the need for extended agitation/flow of wash water altogether. > Long soaks have been demonstrated to cause damage to the > support and are to be avoided. Agreed. > The paper support of "fiber" prints do not wash by a > strictly diffusion process. Ilford showed that the hypo > becomes bound up in the fiber structure of the paper and > comes out by frictional forces, again requiring flowing > water. This process is much slower than diffusion from > gelatin partly accounting for the long wash times required > for fiber support. Again, because stagnant washing does not > produce the mechanical effects needed to remove the hypo it > doesn't work very well. This part is referring to desorption process I mentioned above. A few minutes in washing aid can faciliate the desorption process considerably. > Also, the heroic washing recommended by Vestal, Adams, and > others, was based on the old idea that absolutely no hypo > must remain in the emulsion or support if long image life is > wanted. This was proved untrue by research at Kodak > confirmed by independent research at Fuji c.1961. And if you care that much about the permanence of the image, you'd better toned the image in polysulfide anyway. And if I'm not good enough to tell you (for some), please feel free to ask anyone at the IPI. > Sulfite has a specific ion exchange property for thiosulfate > and silver-thiosulfate complexes thus very much accelerating > the wash rate. This effect is specific to thiosulfate and > its complexes. [...] > I can't speak for alternative processes but Ryuji Suzuki > makes the point that the materials to be washed out in many > of them are not the same as for silver-gelatin so a sulfite > wash aid may be useless. Well, I largely agree with you, but I'd also add that I would not be surprised if sulfite is found to have a similar ionic displacement effect with other ions that adsorb on some substrate molecules. Whether sulfite is sufficiently effective in other "washing" processes to recommend/adapt it in the practice of other processes is a separate question that needs to be tested specifically for that process and materials. > There is a simple test for residual hypo. A solution of > silver nitrate usually preserved with some acetic acid. This > test leaves a stain where there is an excess if hypo. Kodak > has instructions for mixing and using the solution in > several of its handbooks. Because the test spot will > eventually darken with time its necessary to make a judgment > a minute or two after applying the test. A more elaborate > version was also published by Kodak in which the stain if > fixed and stabilized in a sodium chloride solution. This > test can be used for comparison to standards using a > densitometer in order to obtain a quantitative measurement > of residual hypo. For the latter test, Mattey and Henn used sodium chloride and sodium thiosulfate. However, it is further simplified by Pope, replacing those two agents with a mixture of ammonia and sodium chloride, to cut the required test time in half. I sent you a copy of this paper several years ago. It is this: PHOTOGRAPHIC SCIENCE AND ENGINEERING Volume 13, Number 5, September-October 1969 A Simplified Method for Determining Residual Thiosulfate in Processed Microfllm* C. I. POPE, Image Optics and Photography Section, National Bureau of Standards, Washington, D.C. 20234 (abstract) In the silver nitrate test for residual thiosulfate in processed film the silver ion reacts quantitatively with thiosulfate to form silver sulfide in situ. The excess silver ion is removed by treating the sample in solutions of sodium chloride and sodium thiosulfate followed by washing. The method is simplified by using a solution of ammonium hydroxide and sodium chloride to remove the ex- cess silver ion; no washing is necessary. The new method takes half the time previously required. (abstract end; read the whole paper for details) However, in reality, just the old plain silver nitrate with acetic acid is more than adequate for routine quality control in darkroom. > Vertical, so called archival, washers were designed with the > idea that each print would be separated from the others and > a good water flow maintained at both surfaces. Some work OK, > some don't. In general, the rate of exchange of water is too > slow because of the relatively large total volume of these > washers. Kodak recommends that the water in a washer change > in five minutes. Measurements in my Zone VI 16x20 washer > show it doesn't come anywhere near this rate. In order to > maximize the rate I use it in a bathtub with the exhaust > hose removed and flow high enough to cause overflowing. I > also pull the plug and drain it about halfway through the > wash cycle. I tested it by putting some beet juice in it and > seeing how long it took for the dye to dissipate. Do you use wash aid? Do you run the silver nitrate test? I never owned Zone VI washer but my guess is that if you fix, quick rinse, wash aid, quick rinse and then use your washer, you probably do not need to do any of the above to achieve the current ISO standard. > I am unaware of any research on either the most effective > method of washing platinum or other alt processes or tests > for completeness of washing but they may exist. Tests for completeness of washing can be established only after identifying what chemical species are responsible for deterioration. There may be more than one of them, and it may depend on the processes. But one thing I would be concerned is redox-active metal ions, such as Fe(2+/3+) ions, which can create hydroxyl radicals. Hydroxyl radicals are known to disintegrate cellulose fibers. -- Ryuji Suzuki "People are sick in this country because they are poisoned [...] they poison themselves. To my way of thinking why about 90% of people are sick is because they eat shit---Bill Maher (2007)"
|